Super badass varkid - shoot, dont kill, wait till it goes into pod
Ultimate badass varkid - shoot, dont kill, to the best of my knowledge its totally random if ot goes into a pod or not. If it goes into a pod, you get Vermivorous. Otherwise you don't.

Oh btw, if you ever get a chubby varkid at any point, it wont go any further so just kill it.